1. Field of Use
This invention relates generally to a control system for regulating the torque and speed of a wound rotor electric motor such as is used, for example, in the traverse drive of an overhead travelling crane.
2. Description of the Prior Art
In the operation of overhead travelling cranes used in industry, it is desirable to be able to regulate the torque and speed of the traverse drive motor to provide for smooth acceleration, speed regulation, and good inching performance so as to reduce or prevent swinging or jerking of a load suspended from the crane hook at the traverse drive motor is started, stopped, accelerated, decelerated, and during transitions between forward and reverse movement. Heretofore, in some cranes where the traverse wheels were driven by an alternating current wound rotor motor, primary or secondary reactors were employed to regulate power in the motor. Banks of SCRs have also been used to regulate primary power. Direct current choppers have been used to effectively change secondary resistance and thus effect motor control. As implemented, most of these control schemes obtain torque and speed control through similar methods. Torque control has in general really been a current control. For example, in a secondary reactor system, when the motor is plugged, torque was controlled solely by regulating current to the motor. However, current flow in the secondary winding of the motor is not only a function of motor torque but also of the total power supplied to the motor. Control of current without any reference to the power factor leads to a rather nonlinear torque/speed characteristic. In the past it has been up to the operator to compensate for these nonlinearities by manipulation of the operator's control handle to prevent too rapid acceleration or deceleration of the traverse drive, with resultant swinging of the suspended load. However, speed control has been obtained through the use of a DC tachometer, but, while this provides excellent control, it detracts from the simplicity and ruggedness normally desired in an alternating current induction drive. Where torque and speed have been used in conjunction with one another, it has been a simple summing circuit which in general was accompanied by poor regulation.